Existing browsers do not consider whether a certificate was signed by a different CA than before; a laptop that has seen Gmail’s site certified by a subsidiary of U.S.-based VeriSign thousands of times would raise no alarm if Gmail suddenly appeared to present a different key apparently certified by an authority in Poland, the United Arab Emirates, Turkey, or Brazil. Yet such a change would be an indication that the user’s encrypted HTTP traffic was being intercepted.
— New Research Suggests That Governments May Fake SSL Certificates
